1. Field of the Invention
The present invention relates to a technique for forming a trench MOS gate to be applied to a semiconductor device, particularly to a power device. The invention also relates to a device isolation technique.
2. Description of the Background Art
A. Background Art
(a-1) First Background Art and Problems thereof
FIGS. 65 to 72 are cross-sectional views showing a conventional process for forming a trench MOS gate portion in step-by-step fashion. In particular, FIG. 72 is a cross-sectional view when a trench MOS gate portion 131 has been formed.
Referring to FIG. 65, a doped P type region 2 is initially formed on an upper surface of a semiconductor substrate 1 made of Si and the like. A heavily doped N type region 3 is selectively formed in an upper surface of the doped P type region 2. An oxide film 21 is formed on top of the structure thus obtained. Then a trench 4 is formed which extends through the oxide film 21, the doped P type region 2, and the heavily doped N type region 3 (FIG. 65).
A silicon oxide film 7 is formed in the trench 4 (FIG. 66). After the oxide films 7 and 21 are removed (FIG. 67), a silicon oxide film is formed as a gate oxide film 9 (FIG. 68).
An oxide film which is removed immediately after it is formed, such as the silicon oxide film 7, is referred to as a "sacrificial oxide film" hereinafter. For shaping of the trench and removal of defects, strain and contamination in the trench, the sacrificial oxide film is sacrificed without remaining in a structure to be completed later. The silicon oxide film 7 of 100 to 300 nm in thickness is formed in an atmosphere of oxygen at a temperature ranging from 950 to 1100.degree. C., for example.
The gate oxide film 9 is generally formed by thermal oxidation in an atmosphere of steam at a temperature not more than 1000.degree. C. This is because the oxide film formed by thermal oxidation in an atmosphere of steam is, in general, less defective than the oxide film formed by thermal oxidation in an atmosphere of oxygen and because the oxide film is less defective at a lower temperature.
Low-resistance polycrystalline silicon 10, for example, is filled into the trench 4 (FIG. 69) to form a gate electrode 22 in the trench 4. A silicon oxide film 11 is formed on the gate electrode 22 (FIG. 70). A CVD oxide film 12 is formed on top of the structure provided through the foregoing steps (FIG. 71) and is then shaped by etching into the trench MOS gate portion 131 (FIG. 72).
The trench 4 after the silicon oxide film 7 is once formed and removed has a characteristic configuration as illustrated in FIG. 67. That is, an opening portion and a bottom of the trench 4 are of angular configurations 5c and 6c, respectively.
Such configurations of the trench 4 result in a non-uniform thickness of the gate oxide film 9 formed in the trench 4. In particular, the gate oxide film 9 is most pronouncedly thin in positions reflecting a configuration 5d of the opening portion of the trench 4 and a configuration 6d of the bottom of the trench 4.
Such reduced thickness of the gate oxide film 9 in the trench 4 particularly in the opening portion and bottom of the trench 4 results in gate breakdown position and breakdown voltage failures. In addition, a leak current of the gate oxide film 9 increases.
Further, the angular configurations 5c, 6c of the trench 4 deteriorate the characteristics of the trench MOS gate portion 131. In the step of forming the trench 4, defects are prone to occur about the trench 4. The defects deteriorate the characteristics of channels formed when a predetermined potential is applied to the gate electrode 22, and reduces a mobility in an MOS gate channel which is a basic characteristic of a power device having the trench MOS gate portion 131 due to defects, strain and contamination adjacent an MOS gate interface, resulting in a rise in on state voltage.
(a-2) Second Background Art and Problems thereof
FIGS. 73 to 81 are cross-sectional views showing a process for fabricating lateral IGBTs trench-isolated in an SOI (silicon on insulator) structure in step-by-step fashion.
Referring to FIG. 73, substrates 1e and 1d made of silicon and the like are bonded together, with a silicon oxide film 25 therebetween. P layers 41 and N.sup.+ layers 42 are selectively formed in an upper portion of the semiconductor substrate 1e. A silicon oxide film 43 is formed over the semiconductor substrate 1e.
The silicon oxide film 43 is selectively removed so that parts of the P layers 41 and N.sup.+ layers 42 are exposed (FIG. 74), and silicon etching is performed using the remaining silicon oxide film 43 as a mask. This permits the semiconductor substrate 1e to be selectively excavated down to form trenches 44 (FIG. 75).
Then, sacrificial oxide films 45 are once selectively formed on inner walls of the trenches 44 by thermal oxidation (FIG. 76), and the silicon oxide films are etched. This permits the removal of parts of the silicon oxide film 25, all of the sacrificial oxide films 45 and all of the silicon oxide film 43, and also permits the trenches 44 to be further excavated down to the level lower than the bottom of the semiconductor substrate 1e (FIG. 77). Thermal oxidation in an atmosphere of steam at a temperature not more than 1000.degree. C. provides isolation oxide films 46 around the remaining semiconductor substrate 1e (including the P layers 41 and N.sup.+ layers 42) (FIG. 78).
Polycrystalline silicon 47 is deposited over the structure of FIG. 78 to fill the trenches 44 with the polycrystalline silicon 47 (FIG. 79). The polycrystalline silicon 47 over the semiconductor substrate 1e is selectively removed so that the polycrystalline silicon 47 remains only in the trenches 44. The polycrystalline silicon 47 is covered with field oxide films 48. The field oxide films 48 are also formed on the surface of the semiconductor substrate 1e between the P layers 41 and the N.sup.+ layers 42 (FIG. 80). Then a predetermined doped layer is formed, and lateral IGBTs are formed which are isolated from each other by isolating portions 13a having a trench structure (FIG. 81).
Construction of the isolating portions 13a in this manner causes the problems of the thickness of the isolation oxide films 46 similar to the first background art problems. Specifically, as shown in FIG. 78, the semiconductor substrate 1e (including the P layers 41 and N.sup.+ layers 42) is of an angular configuration in opening portions 44a and bottoms 44b of the trenches 44. The isolation oxide films 46 in these portions are pronouncedly thinner than those in other portions. The isolation oxide films 46 are prone to be broken particularly in the bottoms 44b. This causes the problem of a lowered isolation breakdown voltage by the isolating portions 13a.